1. The present invention relates to monolithic articles made from sheets of metal having preferred crystallographic orientations.
2. It has long been recognized by mechanical designers and materials engineers that many structures require different material properties at different locations. A common solution to this problem is to join dissimilar materials, such as by welding or brazing. Another approach has been to process the different parts of the structure in different ways, such as by heat treating one portion of a structure in a different manner from another portion of the structure. It has long been known that mechanical working of metals will induce in the resultant articles metallurgical structures which provide altered properties. This knowledge is often employed to advantage in the hot and cold working of metals to alter their grain structures.
Anisotropic properties are obtained in various wrought, cast and composite materials. Anisotropy is used to advantage in an article if the preferred properties' direction can be aligned with the critical forces to which the article is subject.
Erickson et al in U.S. Pat. No. 4,184,900 "Control of Microstructure in Cast Eutectic Articles" discloses a directionally solidified eutectic article having microstructure which varies along the length of the article, to provide different properties in the root and airfoil of a blade. In the Erickson et al invention, the microstructure is controlled by varying the solidification rate. U.S. Pat. No. 3,598,169 to Copley et al discloses a directionally solidified cast disk which has a crystallographic orientation which varies around the periphery of the disk. U.S. Pat. No. 3,967,355 "Composite Single Crystal Article" granted to Giamei and Kear, describes how a gas turbine airfoil is made by bonding anisotropic single crystal segments. Such segments can be bonded to isotropic polycrystalline materials as well to provide varied properties. All the foregoing patents are assigned to the present assignee.
Thus, it can be said that there are many ways in which the anisotropic properties of a monolithic cast metal article can be tailored to a particular application. However, the aforementioned casting techniques are limited to essentially small articles. Large articles can be made as composite welded assemblies of castings, but they are costly.
Rolling and forging are more suited to large shape manufacture. Still, for axisymmetric structures such as large cylinders and disks which characteristically are heavily used in gas turbine engines, there are few techniques for obtaining desired anisotropic metallurgical structures other than those which naturally result from the conventional forming processes. For instance, in the making of a ring by regular ring-rolling techniques the amount and direction of deformation is limited by commercial equipment and by the interdependence of the ring diameter on the amount of deformation which is applied to the structure as it is expanded.
For high temperature superalloys with which the present invention is particularly concerned, the flexibility in mechanical working and obtaining heat treatment response is often quite limited. One of the reasons for this is that superalloys are by their very nature extremely strong and resistant to deformation. In addition, since they are utilized at high temperatures and under a variety of transient temperature conditions, they must have metallurgical structures which are stable over a wide range. This characteristic limits their response to many conventional processing techniques.